At the present time, the thread is attached to the filter bag and to the pick-up tag by knotting when high quality filter bags are required, differing from similar kinds of bags in; that all the parts of them that come into contact with the infusion cannot have even the least harmful effects on health and (also very important from the commercial viewpoint) cannot alter the taste of the infusion. For example, bad effects on health and taste may be produced when the filter paper forming the bag is sealed using hot-melt glue or when the top of the bag is closed using metal staples which are also used to attach the thread to the bag.
The present invention relates in particular to an improved needle of the type comprising two eyes located at different distances from the needle point, the first eye being designed to hook the thread, stretched and retained at the end sections and to make a loop in the thread when the needle is pulled in the direction of its axis; and the second eye being located closer to the point and oriented at right angles to the first eye so that it can be surrounded by the loop and can guide an end section of the thread across the loop to make a knot. A needle of this type is disclosed in Italian patent application No. BO95 A000J48.
In an improved machine forming the subject matter of another application filed at the same time by the same Applicant, the loop, before being crossed by the section of thread that will make the knot, is also twisted by turning the needle about its axis one or more times, which reduces the size of the loop and gradually tightens it around the lateral surface of the needle.
In both these solutions, apart from the constraints posed by the way the thread is hooked to the eye and the extent of the pulling force exerted by the needle itself, the loop adapts to the surface of the needle quite freely. When the thread is pulled by the needle, the loop assumes a flat configuration. As the loop is twisted, the plane in which it originally lay is gradually distorted and the more the loop is twisted, the more its plane is distorted.
Under certain working conditions, the distortion of the loop against the lateral surface of the needle in this way can result in a not entirely correct position of the loop relative to the second eye of the needle which, as is well known, acts as a guide for special threading elements used to insert an end section of the thread into the loop.
Although this problem can be avoided by accurate setting up and frequent adjustment, there is room to improve machines of this kind so that they maintain the correct setup for a longer time without requiring frequent adjustments.